1. Field of the Invention
The present invention relates to an optical modulator and to an optical short pulse generating device, an optical waveform shaping device, and an optical demultiplexer device using this optical modulator.
2. Description of the Related Art
In the past, DFB (Distributed Feedback)-LDs and other single-wavelength laser diode devices were directly modulated in order to subject optical signals to digital intensity modulation, that is, to form on-off optical signals. A shortcoming of this method, however, is the broad spectrum of the generated light.
In view of this, external modulators are used in place of direct modulation for ultrahigh-speed modulation and for soliton modulation, phase modulation, and time-division multiplexing in the optical region.
Devices utilizing an electro-optical effect and devices utilizing an electro-absorption effect are used as such external modulators.
Optical modulators that utilize the electro-optical effect are devices for performing phase or intensity modulation by utilizing a phenomenon in which the index of refraction of LiNbO.sub.3 or another ferroelectric crystal changes when an electric field is applied to this crystal. A known example is the Mach-Zehnder modulator.
Furthermore, devices that utilize the electro-absorption effect are devices utilizing an effect (Franz-Keldysh effect) in which the absorption-edge wavelength of a semiconductor is shifted in proportion to the square of the electric field, and InGaAsP and the like are used as the semiconductor layers in the 1.5-.mu.m band.
FIG. 6 is a diagram schematically illustrating the structure of such an electro-absorption-type optical modulator 100. In the drawing, an InGaAsP layer 102 (modulation waveguide) is formed on an n-type InP semiconductor substrate 101, semi-insulator InP 103 is positioned on both sides of the InGaAsP layer 102, and a p-type electrode 106 is provided above the InGaAsP layer 102 via a p-type InP layer 104 and a p-type InGaAsP layer 105. In addition, an n-type electrode 107 is provided underneath the n-type InP semiconductor substrate 101, and applying a modulation signal voltage between the n-type electrode 107 and the p-type electrode 106 varies the electric field applied to the InGaAsP layer 102, changes the optical absorption characteristics, and subjects incident light to optical intensity modulation.
Furthermore, the optical transmission characteristics versus the applied voltage of the electro-absorption-type optical modulator are nonlinear characteristics suitable for the direct production of the sech.sup.2 -type optical short pulses needed for optical soliton communications, and an optical short pulse generating device utilizing such an electro-absorption-type optical modulator has already been proposed (Japanese Laid-Open Patent Application 5-283804).
FIG. 7 depicts the structure of such an optical short pulse generating device.. In the drawing, 100 is an electro-absorption-type optical modulator that has the structure described above, 110 is a DFB-LD or other semiconductor laser device for continuously generating single-wavelength light, 111 is a DC bias voltage source for supplying a constant DC bias voltage, and 112 is a sine-wave generator for producing sine-wave signals. Signals obtained by adding the output of the DC bias voltage source 111 and the output of the sine-wave generator 112 are applied as modulation signals between the p-type electrode 106 and the n-type electrode 107 of the electro-absorption-type optical modulator 100.
The operation of an optical pulse generating device thus configured will now be described with reference to FIG. 8. In the drawing, optical transmission characteristics versus the applied voltage of the electro-absorption-type optical modulator 100 are shown at (a) in FIG. 8. It can be seen in the drawing that transmissivity decreases as an essentially exponential function when a negative voltage is applied. The modulation voltage applied to the electro-absorption-type optical modulator 100 is shown at (b) in FIG. 8. The voltage at which the transmissivity (a) is essentially zero is produced as a bias voltage by the DC voltage source 111, and the voltage resulting from the superposition of this voltage with the sine-wave signal outputted from the sine-wave generator 112 is applied between the p-type electrode 106 and the n-type electrode 107. As a result, the optical short pulses shown at (c) in FIG. 8 are outputted from the electro-absorption-type optical modulator 100. These optical short pulses form a waveform that is very close to the soliton condition of 0.315, which is a product of the full width at half maximum of the spectrum and the full width at half maximum of the time waveform of the hyperbolic secant waveform.
It has also been proposed to use the electro-absorption-type optical modulator 100 as a 2R (Retiming/Reshaping) repeater in optical soliton transmission system and an optical demultiplexer for an optical time-division multiplexing system (Japanese Laid-Open Patent Applications 5-284117 and 7-79198).
As described above, electro-absorption-type optical modulators are used in a variety of applications, but one of the properties of such modulators is that the insertion loss has considerable polarization dependence. This dependence is the result of the rectangular cross section (for example, about 1 .mu.m in the crosswise direction and about 0.2 .mu.m in the longitudinal direction) of the InGaAsP layer 102, which serves as a modulation waveguide, and, depending on the plane of polarization of incident light, the insertion loss may vary by as much as about 0.5 dB. This phenomenon presents a very serious problem for a repeater in which, for example, an optical amplifier is used. Specifically, for such an optical amplification repeater system, the polarization dependent loss (PDL) must be, for example, 0.1 dB or less for the entire repeater system and 0.01 dB or less for each component. The tendency for having considerable polarization dependent loss of electro-absorption-type optical modulators therefore presents a very serious problem.
In view of the foregoing, an object of the present invention is to provide an electro-absorption-type optical modulator devoid of any polarization dependent loss.
Another object of the present invention is to provide an optical short pulse generating device, an optical waveform shaping device, and an optical demultiplexer device that involve using the electro-absorption-type optical modulator devoid of any polarization dependent loss.